In recent years, various peptides have attracted attention as a drug candidate or research tool and there have been attempts to develop a peptide library and screen peptides having affinity to a target substance.
For artificial creation of a peptide library, a chemical synthesis method, a method using a biosynthesis enzyme of a secondary metabolite, a method using a translation synthesis system, or the like has conventionally been used.
The chemical synthesis method has difficulty in enhancing diversity of a library. In addition, this method needs much time for screening or for analysis of correlation between the structure and activity of a compound.
The method using a biosynthesis enzyme of a secondary metabolite, on the other hand, permits rapid and easy construction of an elaborate skeleton or chemical conversion. This method is however not suited for construction of a large-scale compound library because kinds of compounds that can be synthesized by this method are limited due to substrate specificity of the enzyme.
When the translation synthesis system is used, a peptide library rich in variety can be constructed in a short time by creating an mRNA library and translating it. In addition, by using it in combination with an mRNA display method or the like, nucleic acid molecules which are genotype molecules and peptides which are phenotype molecules can be correlated to each other, making it possible to rapidly and easily searching and concentrating peptides that bind to a desired target molecule from the library. Thus, using a translation system for the synthesis of a peptide library has many advantages, but it is almost limited to production of peptides composed of a proteinogenic amino acid.
In particular, the present inventors have elucidated that when a peptide is synthesized in in vitro translation system, it is very difficult to incorporate a non-proteinogenic amino acid bearing an electrically charged side chain. Similarly, it is reported that with regard to incorporation of an N-alkyl amino acid in protein by nonsense suppression in a translation system using a cell extract, an incorporation efficiency of a charged N-alkyl amino acid is markedly low (Non-patent Documents 1 to 3) and an incorporation efficiency of a non-charged N-alkyl amino acid is high (Non-patent Documents 4 to 7).
According to recent reports, some non-charged N-alkyl amino acids are suitably used in in vitro peptide selection (Reference Documents 8 to 10), but structural diversity of charged non-proteinogenic amino acids that can be incorporated in a peptide by ribosomal synthesis has still been limited.
It is therefore presumed that the structural diversity of a peptide library that can be used for in vitro peptide selection can be enhanced markedly if a charged non-proteinogenic amino acid is incorporated in a peptide by ribosomal synthesis.